Process for adjusting the components in aqueous alkali cyanide electrolytes



United States Patent Ofiice 2,861,927 Patented Nov. 25, 1958 2,861,927PROCESS FOR ADJUSTING THE COMPONENTS IN AQUEOUS ALKALI CYANIDEELECTROLYTES Myron Ceresa and James R. Crain, Penn Township, AlleghenyCounty, Pa., assignors to Westinghouse Electric Corporation, EastPittsburgh, Pa., a corporation of Pennsylvania No Drawing. ApplicationApril 25, 1956 Serial No. 580,458

4 Claims. (Cl. 204-44) The present invention relates to the treatment ofelectrolytes. and has particular reference to the treatment of aqueousalkali cyanide electrolytes to purify them and to adjust the componentsthereof to achieve improved a great excess which may amount to from 20to 25 ounces of alkali metal carbonate per gallon of electrolyte. Thisis far above the desired carbonate content which usually varies from 4to 8 ounces per gallon, and preferably not greater than 10 ounces pergallon.

Increase of the carbonate content in alkali cyanide electrolytes hasbeen particularly severe in plating installations wherein the rinsewater applied to the plated members to wash off the dragout cyanidesolution is returned to the plating tank. Previously, it was a commonpractice to discard to a drain the rinse water containing theelectrolyte dragout. Thus, in the operation of a conveyorized platingline, as previously practiced, the dragout losses were sufficient toequal a complete turnover of the electroplating electrolyte every 3 to 6months on the average. Such dragout tended to keep the carbonate contentof a cyanide electrolyte at a low level. Consequently, there was noproblem of excessive carbonate build-up as there is today when theelectrolyte dragout is returned to the plating tank.

Also, due to present day high production demands the operatingconditions and components of the electrolyte baths have been changed tomeet desired increased plating rates. Shorter plating times are usedtoday than formerly and this necessitates operating the electrolyte at ahigher plating current density. Higher-plating current densities usuallyrequire increasing the electrolyte temperature, applying more agitationto the electrolyte by air bubbling or by mechanical stirring, andincreasing the concentration of the dissolved components in theelectrolyte. These changes in operating conditions and compositions havebeen important factors in the increased rate at which the carbonateconcentration is built up in cyanide electroplating baths. In a cyanidebath operated with air or mechanical agitation the oxygen and carbondioxide in the air appreciably increase the rate at which the carbonatesbuild up in the bath. Higher operating temperatures and higherconcentrations of the components of the bath, especially the alkalimetal cyanide and alkali metal hydroxide, also increase the rates atwhich these components are converted to carbonates in the cyanideplating bath.

Thus, the present day practice of returning the rinse water to theelectroplating tank and the changes in operating conditions to meethigher production demands has resulted in a great increase in thecarbonate build-up in cyanide electroplating baths.

It is well known that organic impurities tend to accumulate in platingtanks. Such organic impurities have ,been found in many cases to affectdetrimentally'the quality of the electro deposited metal. Such organiccontaminants may comprise material leached out of the tank lining andrack coatings. Air, contaminated with oil and organic dust particles,also contributes a substantial amount of organic impurities to theelectrolyte, particularly where air agitation of the electrolyte isemployed. In some cases organic matter may be added inadvertently or insome cases it may be the result of the decomposition of organic additionagents present in the electrolyte.

In many cases, the work being plated in the electrolyte drags in someamounts of organic and other impurities such as chromates and sulfates.The chromates and sulfates are generally undesirable components andshould be kept at a very low concentration in the alkali cyanide platingbath.

The following reactions have been found to occur in the electrolyte andresult in depletion of certain constitutents of the plating bath whilesimultaneously increasing the carbonate content.

H2O +CO2'9H2CO3 The above equations relate to the reactions of therespective sodium compounds but similar reactions apply to the potassiumor lithium compounds. It will be observed that in Equation 1 sodiumcyanide is depleted, while in Equation 2 sodium hydroxide is depleted.In Equations 3 and 4, sodium cyanide and sodium hydroxide are destroyedby the carbonate forming reaction.

It is well known that there is a specified range of proportions withinwhich the components of an electrolyte must be maintained in order toproduce the best plating results. If any of the components of analkaline cyanide electrolyte such as the alkali metal hydroxide, alkalimetal cyanide, or alkali metal carbonate depart from specification, thenthe plating operation will be less satisfactory and the quality andspeed of plating will suffer in proportion to such departure from thedesired proportions. Inasmuch as most of the carbonate forming reactionsdeplete the alkali metal cyanide and alkali metal hydroxide, there notonly results an undesirable excess of carbonate but also a reduction ofthe free alkali metal cyanide present in the electrolyte and anundesirable change in the alkalinity of the electrolyte.

While it has been proposed in the art to add some one ingredient, suchas gypsum (calcium sulfate), magnesium carbonate, barium hydroxide, orcalcium hydroxide to an electrolyte in order to precipitate magnesium,barium or calcium carbonates therefrom, the treated electrolyte is notin proper balance or adjustment with respect to free cyanide or thehydroxide content. In some cases the addition. of such carbonateremoving agents has required a plurality of succeeding adjustingtreatments of the electrolyte in order to bring it into reasonableoperating balance.

The addition of calcium hydroxide alone, for example, to a platingelectrolyte has been found to result in a number of unsatisfactoryfeatures. In many cases, the calcium carbonate formed thereby is in theform of a colloidal or slimy precipitate that settles out of theelectrolyte extremely slowly and is' dilficult to remove therefrom byfiltration.

In copending application Serial No. 477,424 to Myron Ceresa et al.,which application is assigned to the same assignee as the presentapplication, there is disclosed a (place a portion-of the'calciumcompounds. -to an aqueous electrolyte which is out of specification,

process for treating aqueous alkali cyanide electrolytes which are notin desired operating specification because *That application discloses aprocess for treating such electrolytes --to return them to desiredoperating specification. Broadly, that process comprises treating theaqueous electrolyte with a single composition containing calciumcyanide, calcium oxide, and an alkali metal cyanide. In some casesbarium compounds may re- Whenadded that composition brings abouttheprecipitation of the excess carbonates -an d.. adds free alkali metalcyanide and alkali metal hydroxide whereby the electrolyte is restored-to a desired operating specification.

'The process disclosed in the said copending application is particularlysuitable for use in treating electrolyte plating baths wherein some orall the rinse waters containing the electrolyte dragout are discarded toa drain. In those plating installations wherein the electrolyte dragoutis' reclaimed and-returned to theplating tank there are problems ofattaining a. complete balance of composition using the treating agentsof that application. The reason for-thisis that in installations of thelatter type there is only a moderate or minor loss of free alkali metalcyanide from the plating bath since substantially all the electrolytedragout containing alkali cyanide is returned to the tank. When thecomposition of the copending application containing calcium cyanide isadded to such an electrolyte, the lkali metal cyanide present thereinoften increases the free alkali metal cyanide in the electrolyte above adesired level. As a result, the electrolyte must be subjected toadditional treating steps or to a modified treatment to reduce the freealkali metal cyanide to the desired level before best plating can beobtained.

The object of the present invention is to provide a process for treatingaqueous alkali cyanide electrolyte plating baths wherein the cyanidedragout is reclaimed during use and returned to the bath, the processcomprising adding a composition to the bath to remove excessivecarbonates therefrom and return the bath to within desired operatingspecification.

Still another object 'of this invention is to provide a process fortreating aqueous alkali cyanide electrolyte plating baths wherein thecyanide dragout is reclaimed during use and returned to the bath, theprocess comprising adding a composition to the bath consistingessentially of calcium cyanide and calcium oxide to remove excessivecarbonates therefrom and return the bath to within desired operatingspecification.

A further object of this invention is to provide a process fortreatingaqueous alkali cyanide electrolyte plating baths wherein atleast a substantial proportion of the cyanide electrolyte dragged out ofthe bath during plating is reclaimed and returned to the bath, theprocess comprising treating the bath with predetermined quantities ofcalcium cyanide and calcium oxide to precipitate excessive carbonatesand organic impurities fromthe bath.

Other and further objects of the invention will, in part, be obvious,and will, in part, appear hereinafter.

In the attainment of the foregoing objects and in .accordance with thepresent invention, a process is provided for treating aqueous alkalicyanide electrolytes with a calcium cyanide and calcium oxidecomposition to remove excess carbonates therefrom and increase the freealkali metal cyanide and free alkali metal hydroxide content thereof toa desired predetermined level whereby excellent electroplating results.The treatment can be applied to alkali cyanide baths employed forplating gold, silver, copper, brass, zi nc andcadmium. Theseelectrolyteslwill contain at least 0.5 ounce per. gallon f a kal qxiMore specifically, the process of this invention comprises adding acomposition consisting essentially offrom 9% to 91% by Weight of calciumcyanide and from 91% to 9% by weight of calcium oxide to an aqueousalkali cyanide electrolyte. The composition is added in an amount toprovide sutticient Ca'(CN) and CaO (1)10 react with substantially all oftheexcess carbonate to causethe same to precipitate from the electrolytesolution as calcium carbonate leaving only the specified amount ofcarbonate to meet specification, and (2) increase the free alkali metalcyanide and alkali metal hydroxide content of the electrolyte to adesired predetermined level.

The process of the present invention is adapted to the treatment orpurification of aqueous alkalicyanideelectroplating baths which, throughuse, are out of satisfactory operating specification. More particularly,the process of this invention is adapted to the treatment of electrolyteplating baths in which a substantial proportion, for example, at least5% by weight of the cyanide dragout is re-claimed and returned to theplating tank.

It has been determined that organic impurities are removed from aqueouselectrolytes treated in accordance with this invention without the needof adding an additional treating agent. It has been found that theorganic impurities are removed, apparently by being occluded by thecalcium carbonate precipitate and thereby removed from the electrolyte.

Aqueous alkali cyanide electrolyte plating baths sometimes arecontaminated with relatively small amounts of chromates and sulfates. Ithas been determined that these contaminants may be removed from theplating bath by incorporating a relatively small amount, for example,from 0.01% to 5% by weight of barium cyanide with'the compositionconsisting essentially of calcium cyanide and calcium oxide and addingthe combined composition ,to the bath. The chromates and sulfates willprecipitateas barium salts with the carbonate precipitate and may beremoved from the bath in accordance with conventional procedures.

Thus, the invention provides a process for treating aqueous alkalicyanide electrolytes, which electrolytes have at least 5% by weight ofthe cyanide dragout .returned thereto, to remove therefrom excesscarhonate, organic impurities, chromates and sulfates, and provides amethod for increasing the free alkali metal cyanide and alkali metalhydroxide content thereof to a desired predetermined level. Such aprocess rapidly and effectively returns the electrolyte to withinoperating specification by adding thereto a composition consistingessentially of calcium cyanide and calcium oxide together with a minoramount of barium cyanide. It will be understood that calcium hydroxidemay be used in place of all ,or a portion of the calcium oxide.

The two essential ingredients forming the electrolyte treatingcomposition of this invention may be introduced into the electrolyteeither jointly or separately, in the form of solids or completely orpartially dissolved in water, preferably deionized water. While the useof chemically pure ingredients is preferred, it has been determined thatthe presence of minute amounts ofcarbon, metal oxides, halogen salts andthe like may be present in the compositions.

in applying the compositions of this invention to aqueous alkali cyanideelectrolytes, it is desirable that the electrolyte be at a temperatureof not over 200 F., and preferably about to F.

In order to indicate even more fully the advantages and capabilities ofthe present invention, the following examples are set forth. The partsindicated are by weight unless otherwise indicated. In each of theseexamples the Plat n s l n were m lo e n u h m nn r her t l a t 5% by weh f th iIQ Y -F I?S9ut wast claimedand returned to the tank.

EXAMPLE I Chemical analysis-0z./gal.

Time, Hours Free KOH K2003 Copper KCN The cyanide copper platingsolution, operated in the above manner, consumed per week 0.44 oz./ gal.of free KCN and 0.65 oz./ gal. of KOH and formed 0.95 02/ gal. Of K2CO3.

After one week of operation, the free KCN and KOH in the platingsolution were approaching the lower limits and the K CO the upper limitof satisfactory operating specification. In order to continue to obtainthe optimum plating results, the plating solution in the tank wastreated by adding thereto 2 pounds 10 ounces of a dry mixture consistingof 54 percent calcium oxide and 46 percent calcium cyanide. The drymixture was added to the plating solution at l40160 F. About one halfpound activated carbon was also added to the solution. After stirringthe solution for 2 hours, the carbonate precipitate was permitted tosettle and was removed from the solution. The treated cyanide copperplating solution then Was found to contain:

Oz./gal. Free KCN 1.48 KOH 5.20 K CO 8.86 Copper 6.72

The cyanide copper plating solution had been returned to a betteroperating condition, within specification, by this treatment.

Satisfactory results also are obtained by adding the calcium cyanideportion of the treating composition first and then adding the calciumoxide. The two need not be added simultaneously.

EXAMPLE II Periodically throughout a twelve week period, a productioninstallation of 16,500 gallons of cyanide copper plating solution wasanalyzed to determine what change had occurred in the quantities of thevarious components in the solution. It was determined that 0.37 oz./gal.of free KCN and 0.62 oz./ gal. of KOH were consumed per week and thatcarbonate build-up per week amounted to 0.95 oz./gal. To maintain thiscyanide copper plating solution Within desired operating specification,it was necessary totreat a 1000 gallon quantity of the solution once aweek with 800 pounds of a composition consisting of 60 percent by weightof calcium oxide and 40 percent by weight of calcium cyanide. Aftervigorously stirring the mixture for 3 hours, the precipitate whichformed was allowed to settle and the 1000 gallons of treated solutionwas filtered back into the plating tank. Treatment of the 1000 gal.portion of plating solution in this manner lowcred the carbonate contentof the entire solution by 1.04 oz./gal./week and increased the free KCNand KOH content by 0.37 oz./gal./week and 0.61 oZ./gal./weekrespectively, whereby the entire 16,500 gal. solution was returned toand maintained within desired specification.

After 10 weeks of further operation with this production cyanide coppersolution, it was determined that the rate at which the components werebeing consumed and carbonates increased had changed again. Thus, it wasdetermined that each week 0.19 oz./ gal. of free KCN and 0.74 oz./gal.of KOH were being consumed and that the carbonate was building up at arate of 1.17 oz./gal. To maintain the solution in desired operatingspecification, it was necessary to treat 1000 gallon quantities onceeach Week with 700 pounds of a mixture consisting of 76 per cent calciumoxide and 24 percent calcium cyanide. After each weeks treatment,Whether the calcium cyanide and calcium oxide were both added at thesame time or successively, it was found that the carbonate content wasreduced by 1.09 oz./gal. and that the free KCN content was increased by0.22 oz./gal. and the KOH by 0.79 oz./gal. The solution was maintainedin. desired operating specification by this weekly treatment.

EXAMPLE III Time, Hours Free KOH K 003 Copper KCN Thus, this cyanidecopper plating solution consumed per week 0.64 oz./gal. of free KCN and0.31 oz./gal. of KOH. The carbonate content increased at a rate of 0.52oz./gaI./week.

The plating solution in the tank was returned to a desired operatingrange by adding thereto 2 pounds of a mixture of a compositioncontaining 71% calcium cyanide and 29% calcium oxide. This treatmentresulted in the plating solution having the following composition:

Oz./gal.

ree KCN 1.98 KOH 5.00 K CO 6.75 Copper 7.20

It will be understood that this solution will deviate from desiredspecification during each week of use. As indicated above, it may bereturned to desired specification simply by the weekly treatment withtwo pound quan tities of the described additive composition.

EXAMPLE IV Fifty gallons of cyanide copper plating solution wasmaintained at F. in a covered plating tank for 113 hours. The solutionwas not agitated during this period nor was it used to plate anyarticles. The solution was merely maintained at 180 F. to illustrate theeffect of heat on such a solution with respect to carbonate buildup andthe reduction in free cyanide and hydroxide.

Chemical analysis-ozJ gal.

Time, Hours Free KOH K 003 Copper Based. on these data, at the end ofone week during which the solution is maintained at 180 Ftand notoperated, the solution was losing free KCN and KOH at the rate of 0.04oz./gal./week and 0.45 oz./gal./week, respectively, and carbonate wasbuilding up at the rate of 0.40 oz./ gaL/ week. This means that duringdown time of the plating tank, when the plating solution is only beingmaintained at operating temperature, proper treatment with a compositionconsisting of 90 percent calcium oxide and percent calcium cyanide wouldreestablish the solution to within operating specification. Thus, atreatment similar to the one in Example I, viz., a once a Week treatmentwith one pound of a mixture consisting of 90 percent calcium oxide and10 percent calcium cyanide would add 0.04 02/ gal. of free KCN and 0.44oz./ gal. of KOH andremove 0.49 oz./gal. of K CO EXAMPLE V Fifty gallonsof a cyanide copper plating solution in a covered tank maintained at 180F. was agitated by passing nitrogen therethrough at a rate of 0.011cubic foot per gallon per minute. This solution was operated for 200hours according to the manner described in Example I. It was determinedthat 0.64 oz./ gal. of free KCN and 0.10 02/ gal. of KOH were consumedafter one week of operation. The carbonate content increased 0.59 oz./gal. at the end of one week. It was observed that agitation of thesolution with nitrogen gas brought about a relatively large consumptionof free KCN with a relatively small consumption of KOH. It also wasevident that the cyanide was not completely converted to carbonate bythe reactions taking place in the cyanide copper plating solution.Rather, it is believed that the nitrogen accelerated the hydrolysis ofcyanide to oxalates and. formates according to the reactions illustratedin Equation 3 hereinabove. In order to control a solution operated underthese conditions, it is necessary to treat the solution, twice eachweek, with one pound of a composition consisting of 91% calcium cyanideand 9% calcium oxide. With-a total treatment of two pounds each week ofthe additive composition, the free KCN and KOH willbe increased 0.66oz./gal. and 0.07 oz./gal., respectively, and the carbonate will bereduced 0.79 oz./gal.

EXAMPLE VI A 9000 gallon production cyanide zinc plating solution wasanalyzed periodically during operation as a still bath. The analyticaldata indicated that 0.25 oz./ gal. of sodium hydroxide and 0.18 oz./gal. of free sodium cyanide were being consumed with each week ofoperation. it was also determined that the sodium carbonate was buildingup at the rate of 0.40 oz./gal. per week. To maintain this cyanide zincplating solution within operating specification, it was necessary totreat 1000 gallons of the plating solution each week with 270 pounds ofa mixture consisting of 60% calcium oxide and 40% calcium cyanide. Thistreatment removed, eachweek, 0.52 oz./gal. of sodium carbonate and added0.25 oz. /gal. of sodium hydroxide and 0.18 oz./ gal. of sodium cyanidein the entire solution. This treatment maintained the solution withinoperating specification.

While thepresent invention has been described with particularreferenceto the preferred embodiments thereof, it will be understood thatchanges, substitutions, modia fications and the likemay be made thereinwithout departing from its true scope.

We claim as our invention:

1. In the process of plating articles in an aqueous alkali cyanideelectrolyte plating bath containing at least 0.5 ounce per gallon ofalkali hydroxide in which excess carbonate builds up and in whichdeficiencies of free alkali metal cyanide and alkali metal hydroxidedevelop during use, which process includes the step of returning theelectrolyte dragout to the bath, the improvement which comprises addingto the aqueous electrolyte a composition consisting essentially of from9% to 91% by weight of calcium cyanide and from 91% to 9% by weight ofcalcium dioxide, said composition being added in an amount sufficient toprecipitate the excess carbonate, there remain ing at least about 4ounces per gallon of carbonate in the electrolyte, and increase thefreealkali metal cyanide and alkali metal hydroxide to a desiredpredetermined level, said composition maintaining the pH of theelectrolyte at a level of at least 0.5 ounce per gallon of alkalihydroxide.

2. The process as set forth in claim 1 in which the calciumcyanidecomponent and calcium oxide component of said composition areadded to the electrolyte separately.

3. The process as set forth in claim 1 in which the calcium cyanidecomponent and calcium oxide component of said composition are added tothe electrolyte simul taneously.

4. In the process of plating articles in an aqueous alkali cyanideelectrolyte plating bath containing at least 0.5 ounce per gallon ofalkali hydroxide in which excess carbonate builds up and in whichdeficiencies of free alkali metal cyanide and alkali metal hydroxidedevelop during use, which process includesthe step of returning at least5% by weight of the electrolyte dragout to the bath, the improvementwhich comprises adding to the aqueous electrolyte bath a compositionconsisting essentially of from 9% to 91% by weight of calcium cyanideand from 91% to 9% by weight of calcium oxide, said composition beingadded in an amount (1) to provide suflicient calcium cyanide to reactwith substantially all the excess carbonate to form a precipitate ofcalcium carbonate, there remaining at least about 4 ounces per gallon ofcarbonate in the electrolyte, and (2) to increase the free alkali metalcyanide and alkali metal hydroxide to a desired predetermined level,said composition maintaining the pH of the electrolyte at a level of atleast 0.5 ounce per gallon of alkali hydroxide, agitating theelectrolyte and the added composition until the precipitate formingreaction is complete, and then separating the precipitate from. thetreated electrolyte.

References Cited in the file of this patent UNITED STATES 'PATENTS OTHERREFERENCES Transactions Electrochem. Soc., vol. (1941), pp. 358-359.

row 1,.

1. IN THE PROCESS OF PLATING ARTICLES IN AN AQUEOUS ALKALI CYANIDEELECTROLYTE PLATING BATH CONTAINING AT LEAST 0.5 OUNCE PER GALLON OFALKALI HYDROXIDE IN WHICH EXCESS CARBONATE BUILDS UP AND IN WHICHDEFICIENCES OF FREE ALKALI METAL CYANIDE AND ALKALI METAL HYDROXIDEDEVELOP DURING USE, WHICH PROCESS INCLUDES THE STEP OF RETURNING THEELECTROLYTE DRAGOUT TO THE BATH, THE IMPROVEMENT WHICH COMPRISES ADDINGTO THE AQUEOUS ELECTROLYTE A COMPOSITION CONSISTING ESSENTIALLY OF FROM9% TO 91% BY WEIGHT OF CALCIUM CYANIDE AND FROM 91% TO 9% BY WEIGHT OFCALCIUM DIOXIDE, SAID COMPOSITION BEING ADDED IN AN AMOUNT SUFFICIENT TOPRECIPITATE THE EXCESS CARBONATE, THERE REMAINING AT LEAST ABOUT 4OUNCES PER GALLON OF CARBONATE IN THE ELECTROLYTE, AND INCREASE THE FREEALKALI METAL CYANIDE AND ALKALI METAL HYDROXIDE TO A DESIREDPREDETERMINED LEVEL, SAID COMPOSITION MAINTAINING THE PH OF THEELECTROLYTE AT A LEVEL OF AT LEAST 0.5 OUNCE PER GALLON OF ALKALIHYDROXIDE.